This invention relates to an ADPCM (adaptive differential pulse code modulation) transcoder which is for use in code converting between two PCM streams and one adaptive differential PCM stream, namely, which is for code conversion from two PCM streams to one adaptive differential PCM stream and from one adaptive differential PCM stream to two PCM streams.
Such an adaptive differential PCM transcoder is used in an adaptive differential PCM signal transmission network together with other similar adaptive differential PCM transcoders. When taken into consideration, one of the adaptive differential PCM transcoders of the transmission network will be called a local transcoder. Other adaptive differential PCM transcoders are remote transcoders. The local transcoder is for transmitting an adaptive differential PCM stream to a counterpart transcoder of the remote transcoders and for receiving an adaptive differential PCM stream from the counterpart transcoder.
In the manner known in the art, a plurality of baseband signals are multiplexed into each PCM stream. The baseband signals may comprise a voice or voice sound signal and a data or modem signal in an audio frequency band. The modem signal in the audio frequency band will herein be referred to merely as a data signal. Each PCM signal carries the voice signal as a voice or voice signal component. Similarly, the data signal is carried as a data or data signal component.
When supplied to the local transcoder, the two PCM streams will be called first and second input PCM streams. When transmitted from the local transcoder towards the counterpart transcoder, the adaptive differential PCM stream will be called an outgoing adaptive differential PCM stream. When received from the counterpart transcoder, the adaptive differential PCM stream will be called an incoming adaptive differential PCM stream. When converted by the local transcoder from the incoming adaptive differential PCM stream, the two PCM streams will be called first and second output PCM streams.
According to CCITT (International Telegram and Telephone Consultative Committee) Recommendation G.761, each of the first and the second input and output PCM streams carries the baseband signals, thirty channels in number. Each of the outgoing and the incoming adaptive differential PCM streams carries the baseband signals, sixty channels in number. Each of the first and the second input and output PCM streams and the outgoing and the incoming adaptive differential PCM streams has a bit rate of 2048 kbit/s. Each time slot of the first and the second input and output PCM streams consists of eight bit. Each time slot of the outgoing and the incoming adaptive differential PCM streams consists of four bits per channel.
It is known in the art that an adaptive differential PCM transcoder comprises, when used on a send side, an adaptive differential PCM encoder device for encoding two PCM sequences into one or single adaptive differential PCM sequence. A format transforming device is used in format transforming the two input PCM streams into the two PCM sequences and the single adaptive differential PCM sequence into the outgoing adaptive differential PCM stream. When used on a receive side, the adaptive differential PCM transcoder comprises an adaptive differential PCM decoder device for decoding one or sole adaptive differential PCM sequence into two decoded PCM sequences. The format transforming device is used in format transforming the incoming adaptive differential PCM stream into the sole adaptive differential PCM sequence and the two doceded PCM sequences into the two output PCM streams.
It is usual in the art to refer to a combination of an encoder and a decoder as a codec. As a consequence, a combination of the encoding device and the decoding device is herein called a codec device.
A combination of the adaptive differential PCM codec device and the format transforming device is for carrying out the code conversion between two PCM streams and one adaptive differential PCM stream by using an adaptive differential PCM coding algorithm. It is preferred to use an adaptive differential PCM coding algorithm defined by CCITT Recommendation G.721. This, however, makes it impossible to excellently transmit as an adaptive differential PCM stream a data signal having a bit rate which is higher than 9600 b/s.
In order to make it possible to deal with a data signal having a bit rate higher than 9600 b/s, the code conversion is often carried out by using an adaptive differential PCM coding algorithm which is developed individually by a manufacturer of adaptive differential PCM transcoders. This, however, renders the adaptive differential PCM transcoder incompatible with adaptive differential PCM transcoders manufactured by other makers.
Incidentally, the adaptive differential PCM coding algorithm makes use of a scale factor, which is a coefficient used on determining quantization step sizes used on code converting between two PCM streams and one adaptive differential PCM stream. The scale factor is preferably calculated according to adaptive prediction. In such an event, the scale factor is variable depending on a variance which each baseband signal has.
At any rate, a conventional adaptive differential PCM transcoder is defective in that it gives an adaptive differential PCM signal transmission network an objectionable transmission characteristic when the Recommendation G.721 is resorted to. The conventional adaptive differential PCM transcoder is moreover defective in its compatibility with adaptive differential PCM transcoders manufactured by others when the transcoder in question is operable in compliance with an adaptive differential PCM coding algorithm developed by a particular manufacturer.